1. Field of the Invention
The present invention pertains to a high pressure sodium vapor lamp having unsaturated vapor pressure type characteristics with improved lamp voltage fluctuation characteristics.
2. Description of the Prior Art
High pressure sodium vapor lamps have heretofore been known which typically utilize luminescence caused by electric discharge in a high pressure sodium vapor. An arc tube for use with this type of high pressure sodium vapor lamp is commonly exposed to the high temperature sodium vapor during a burning operation. For this reason, an alumina ceramic tube is generally used which has an adequate capacity to endure the effects of the high temperature sodium vapor and yet which exhibits a transmission factor of 90% or more with respect to visible light. In addition to sodium and mercury, xenon as a starting gas is enclosed in the interior of the alumina ceramic tube.
Referring to FIG. 1 which illustrates an arc tube 10 for use in a conventional high pressure sodium vapor lamp, a translucent alumina ceramic tube 1 has opposite end caps 2 and 3 made of alumina, the end caps 2 and 3 being respectively hermetically bonded to both ends of the tube 1 by means of frit. Through holes formed in the centers of the respective end caps 2 and 3 receive electrode support tubes 4 and 5 made of niobium, the tubes 4 and 5 being likewise hermetically bonded to the through holes by means of frit. Electrodes 6 and 7 are secured to the inner ends of the electrode support tubes 4 and 5. The electrode support tube 4 used as an exhaust tube is provided with an exhaust hole 4a while the electrode support tube 5 on the non-exhaust side is provided with a passage hole 5a for preventing air from being sealed within the tube 5. Prior to the sealing of the electrode support tube 4 used as an exhaust tube, a sodium amalgam 8 is enclosed therein.
The arc tube 10 having the above-described structure is, as shown in FIG. 2, supported by support wires 12 or the like in an outer bulb 11, thereby constituting a high pressure sodium vapor lamp 20. An auxiliary conductor 13 serving as an external start-up element is additionally disposed around the periphery of the arc tube 10, and bimetal switches 14 and 15 function to break the electrical continuity of the auxiliary conductor 13 during the burning operation of the lamp.
However, while such a high pressure sodium vapor lamp is burning, the enclosed sodium may react with the translucent alumina arc tube, the sealing frit for the arc tube or an emitter disposed on its electrode. Alternatively, the enclosed sodium may be physically adsorbed by these elements or diffused in the frit. In consequence, the sodium which contributes to the luminescence of the lamp progressively disappears. In order to compensate for this loss of sodium, it has been general practice for the amalgam enclosed in the arc tube to be composed of sodium and mercury in amounts above what would normally contribute to the luminescence, thereby constituting a high pressure sodium vapor lamp of a saturated vapor pressure type.
As described above, such high pressure sodium vapor lamps of the saturated vapor pressure type include an excessive amount of enclosed sodium amalgam. Therefore, if a lamp of this type is disposed in a place susceptible to vibration or in equipment having an airtight structure, factors such as vibration or temperature rise lead to an increase in the sodium vapor pressure and the consequent disadvantage of variations in the lamp characteristics. As regards the ballast that is used in a lighting circuit, both the type which is dedicated for use with high pressure sodium vapor lamps and the low-price ballast utilized in mercury vapor lamps may be employed. In the latter case, however, since only the use of a choke type ballast is allowed, it is impossible to employ a constant power consumption type of ballast having a simple construction in which a capacitor is connected in series. More specifically, although high pressure sodium vapor lamps generally exhibit a rise in lamp voltage toward the end of their service life, use of the constant power consumption type of ballast means that the consumption of power rises as the lamp voltage increases. Since this phenomenon involves excessive power input which might entail breakage of the lamp, it is impossible to use the constant power consumption type of ballast.
On the other hand, an unsaturated vapor pressure type of high pressure sodium vapor lamp is known which enjoys a range of usage that is not limited to particular kinds of equipment and which can be utilized in locations that are susceptible to vibration. In addition, a lamp of this type can be used not only with a choke type ballast of the kind used for mercury vapor lamps but also with a constant power consumption type of ballast. A typical high pressure sodium vapor lamp of the unsaturated vapor pressure type is constructed in such a manner that a limited amount of amalgam composed of sodium and mercury is enclosed in the arc tube and, while the lamp is burning, it assumes a state wherein all the sodium amalgam enclosed therein is vaporized.
Since, however, the amount of sodium enclosed in the arc tube of the unsaturated type high pressure sodium vapor lamp is, as described above, only sufficient to allow for vaporization of all the sodium while burning, the sodium progressively disappears during the burning operation of the lamp, thereby leading to a drop in the lamp voltage. In addition, the influence exerted on the lamp voltage in this way is remarkable as compared with that of the previously-described saturated type high pressure sodium vapor lamp.
When the sodium in the arc tube is completely gone, the mercurcy alone emits light, that is, the arrangement substantially equals that of a high pressure mercury vapor lamp. The lamp voltage is increased to about the level of the initial lamp voltage. However, at an intermediate point before the sodium is completely gone, i.e., while a trace of sodium is still present, the lamp voltage reaches a minimum value. If the lamp voltage drops by an excessive degree when reaching this minimum value, there is a problem in that there is a risk of the ballast burning.